Presently, research continues on developing apparatus for exposing very high resolution hard copies of electronically recorded images. To produce a high resolution hard copy of an electronically recorded image requires that a very large number of closely spaced physically small pixels be exposed on an image recording medium. A PD&C apparatus must accurately position a writing beam on the image recording medium so that the writing beam precisely exposes, on a pixel by pixel basis, every pixel on the image recording medium. Any mispositioning of the writing beam by the PD&C apparatus could result in the misexposure of pixels which would yield an undesirable hard copy image.
The PD&C apparatus in present imaging systems typically employs a discrete encoding scheme and a second light beam to position a writing beam onto the image recording medium. An example of this approach is disclosed in U.S. Pat. No. 4,952,946 issued to Clark et at. on August 1990 and which is incorporated herein by reference. In this patent, a second light beam, known as either a reference or tracking beam, moves in conjunction with the writing beam. Specifically, this second beam is reflected off an encoding device while the writing beam exposes the underlying image recording medium through a scanning slot located adjacent to and along the encoding device. The encoding device which is either a Fresnel mirror, a holographic encoder, or an alternating plurality of reflective and non reflective areas, transforms the light from the second light beam into modulated pulses and reflects these modulated pulses onto a detector and ultimately to the PD&C apparatus. The change in amplitude of these modulated pulses indicates where on the encoding device the second beam is located and, hence, where the writing beam is located on the image recording medium. Accordingly, from these modulated pulses, the PD&C apparatus controls the energization and/or the modulation of the writing beam.
Although adequate for positioning the writing beam, this approach does have some shortcomings. First, this approach may not provide sufficiently precise positional information regarding where the writing beam is located on the image recording medium because of its inability to precisely locate the reference beam on the encoder. This occurs, typically, because the diameter of the reference beam is substantially smaller than the width of a reflective or non reflective area on the encoder. Thus, although the PD&C apparatus can determine in which reflective or non reflective area the reference beam is located, it cannot determine where within this area the beam is located.
Second, this approach cannot determine the direction in which the writing beam travels on the image recording medium. This occurs because the encoder is comprised of only two different encoding states, namely, reflective and non reflective. Thus, regardless of which direction the reference beam travels on the encoder, the same repeating pattern of modulated pulses will be reflected to the PD&C apparatus.
Third, this approach may be prone to degradation in its ability to accurately position the writing beam onto the image recording medium. Since only the second beam provides the position information regarding the writing beam's location on the image recording medium, a strict positional correlation must exist between the second beam's position on the encoder and the writing beam's position on the image recording medium. This strict positional correlation is maintained by a precisely toleranced mechanical connection between those elements that make up the writing beam and those elements that make up the second beam. However, with prolonged use, this mechanical connection could degrade and correspondingly adversely effect the positioning of the writing beam on the image recording medium.
Fourth, this approach is also inappropriate for PD&C apparatus which employ a writing beam that moves along two orthogonal axes. That is, this approach can only accurately position a writing beam that moves along one axis, but not one that moves along two orthogonal axes. This occurs because the encoder is comprised of only two different encoding states, namely, either reflecting or non reflecting and, in order to provide two dimensional positional information which allows for reverse motion, an encoder must have at least nine different encoding states.
Consequently, for all the above mentioned reasons, a need still exists in the high resolution imaging area for a PD&C apparatus that can quickly yet consistently and accurately expose every very small pixel in an image recording medium in order to produce a high resolution hard copy.